This application is a Continuation of International application Ser. No. PCT/JP02/01876 filed on Feb. 28, 2002.
The present invention relates to a process for producing a tetrafluoroethylene polymer (hereinafter referred to as PTFE) which is stretchable at a high stretch ratio and which has high break strength under stretching.
PTFE is used in various applications including various molding materials, highly stretchable low density tapes, uniform high strength stretchable porous materials, binders for fuel cells, etc.
PTFE can be produced by aqueous dispersion polymerization of tetrafluoroethylene (hereinafter referred to as TFE) and can be obtained in the form of an aqueous dispersion having fine PTFE particles dispersed, or can be obtained in the form of a fine powder by coagulating the aqueous dispersion, followed by drying.
Particularly, in its application to e.g. a low density tape, a porous material or a binder for a fuel cell, it is required to be uniformly stretchable to a high stretch ratio, and the obtained low density tape or stretched porous material is required to be uniform and have a high strength characteristic. In its application to a binder for a fuel cell, a further characteristic is required such that it can readily be fibrillated.
To realize such desired properties, various conditions for polymerization of TFE have been studied, and various processes for producing PTFE have been proposed. However, there has been no proposal relating to the purity of TFE to be used for the polymerization of TFE.
An object of the present invention is to provide a process for producing PTFE which is uniform and stretchable at a high stretch ratio and which is excellent in the fibrillation property and has a high break strength.
The present invention provides a process for producing PTFE, which comprises polymerizing TFE in the presence of an aqueous medium and a polymerization initiator to obtain PTFE, wherein the content of impurities in TFE to be subjected to the polymerization is not more than 100 ppm. Further, the present invention provides the above process for producing PTFE, wherein the impurities comprise a saturated compound and an unsaturated compound, the content of the saturated compound is not more than 50 ppm, and the content of the unsaturated compound is not more than 1 ppm.